Burner assembly for delivery of specified heat flux profiles in two dimensions

ABSTRACT

A burner is provided which includes a plurality of burner subunits that each share a single air supply, a single fuel supply and a single control system. Each burner subunit has a plurality of air orifices and a plurality of fuel orifices of sufficient quantity and of a cross-sectional area to control a transverse heat flux profile of the burner. The burner subunits are spaced with respect to one another to control a longitudinal heat flux profile of the burner. The single air supply and said single fuel are adapted to provide an air-fuel mix that ensures the transverse and the longitudinal heat flux profiles are maintained at different fuel and air input rates. A burner of similar design using premixed air and fuel is also disclosed.

BACKGROUND OF THE INVENTION

[0001] The present invention is directed to gas fired burners. Inparticular, the present invention is directed to gas fired burners ofthe type which may be used in industrial furnaces and the like.

[0002] U.S. Pat. No. 5,993,193 (Loftus et al.) discloses a gas firedburner for use in applications such as chemical process furnaces forprocess heaters in refineries and chemical plants. The burner isprovided with a plurality of fuel gas inlets for enabling manipulationof the flame shape and combustion characteristics of the burner basedupon variation in the distribution of fuel gas between the various fuelgas inlets. This invention is directed to varying the pattern of heatflux being produced when the burner apparatus is in operation. However,the invention here is directed to a circular burner with intricatedesign aimed at achieving a great degree of premixing and reduced NOxemissions. More importantly, the heat flux pattern here is thelongitudinal heat flux distribution along the flame. This disclosuredoes not teach heat flux distribution across the burner opening,perpendicular to the flow of flue gas immediately outside the burneropening.

[0003] U.S. Pat. No. 5,295,820 (Bicik et al.) teaches a linear burnerwith jets extending through an opening made in a wall of a body of theburner defining an air-distribution chamber. The jets are connected to aseries of tubes for supplying fuel gas or a gas/air mixture with thetubes passing through the body of the burner in order to be connected onthe outside to a distribution housing provided with gas or with agas/air mixture. The housing has a means to selectively supply the tubesjoined to the jets. The intent here is to have a burner with a widerange of heating power, or turndown ratio. However, this invention doesnot teach a single air supply, single fuel supply, and single burnercontrol system so as to simplify the design and reduce costs whileachieving an object of a desired heat release profile dictated byprocess requirements.

[0004] Additionally, there are arrangements of a multitude of burners infurnaces that achieve a uniform heat flux at a given elevation and agiven heat flux profile along the elevation, such as in a side-firedreformer or a terraced-wall reformer, generally known in the art.However, these burners are individually controlled. They do not share acommon fuel supply manifold or a common air supply manifold. As burners,they are not able to deliver specified heat flux profiles in twodimensions simultaneously. In addition, their cost is usually very highbecause of the need for individual controls.

[0005] It would be desirable to have a burner design that would meetspecified heat flux profiles in two dimensions (e.g., longitudinal andtransverse dimensions) simultaneously. It would also be desirable forthe above to be achieved while meeting safety, flame stability, andlow-cost requirements.

BRIEF SUMMARY OF THE INVENTION

[0006] In a first preferred embodiment, a burner is provided whichincludes a plurality of burner subunits. The burner subunits share asingle air supply, a single fuel supply and a single control system.Each burner subunit has a plurality of air orifices and a plurality offuel orifices. The plurality of air orifices and the plurality of fuelorifices are of sufficient quantity and each air orifice and each fuelorifice is of a cross-sectional area to control a transverse heat fluxprofile of the burner. The burner subunits are spaced with respect toone another to control a longitudinal heat flux profile of the burner.The single air supply and the single fuel supply provide an air-fuel mixthat ensures that the transverse heat flux profile and the longitudinalheat flux profile are maintained at different fuel and air input rates.

[0007] Each of the plurality of burner subunits may be spaced atvariable spacing with respect to one another to control the longitudinalheat flux profile. Alternatively, each of the plurality of burnersubunits may be spaced at a constant distance with respect to oneanother, where each of the subunits have different heat release rates,to control the longitudinal heat flux profile. Alternatively still, eachof the plurality of burner subunits may be spaced at either variablespacing or constant spacing with respect to one another to control thelongitudinal heat flux profile.

[0008] Each of the plurality of burner subunits may have a plurality ofair orifices of a desired cross-sectional area where each air orifice isadapted to create a flamelet to control the transverse heat flux profileof the burner.

[0009] In another preferred embodiment of the present invention, aburner is provided which also includes a plurality of burner subunits.The burner subunits share a single air/fuel supply and a single controlsystem. Each burner subunit has a plurality of air/fuel orifices wherethe plurality of air/fuel orifices are of sufficient quantity and eachair/fuel orifice is of a cross-sectional area to control a transverseheat flux profile of the burner. The burner units are spaced withrespect to one another to control a longitudinal heat flux profile ofthe burner. The air/fuel supply provides an air-fuel mix that ensuresthat the transverse heat flux profile and the longitudinal heat fluxprofile are maintained at different fuel and air input rates.

[0010] Each of the plurality of burner subunits may be spaced atvariable spacing with respect to one another to control the longitudinalheat flux profile. Alternatively, each of the plurality of burnersubunits may be spaced at a constant distance with respect to oneanother, where each of the subunits have different heat release rates,to control the longitudinal heat flux profile. Alternatively still, eachof the plurality of burner subunits may be spaced at either at variablespacing or constant spacing with respect to one another to control thelongitudinal heat flux profile. Each of the plurality of burner subunitsmay have a plurality of air/fuel orifices of a desired cross-sectionalarea where each air/fuel orifice creates a flamelet to control thetransverse heat flux profile of the burner.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0011]FIG. 1 is a simplified cross-sectional side view of a cylindricalsteam reformer in accordance with the present invention.

[0012]FIG. 2 is a simplified cross-sectional view of the steam reformerof FIG. 1 taken substantially along lines 2-2 of FIG. 1.

[0013]FIG. 3 is a schematic diagram of a burner subunit for use in thereformer of FIG. 1 with variable lengths of flamelets and heat transfertargets.

[0014]FIG. 4 is a simplified view of one quarter of a fuel orificearrangement and air orifice arrangement used in the burner subunit ofFIG. 3.

[0015]FIG. 5 is a simplified side elevation view of the reformer of onehalf of the reformer of FIG. 1, depicting an example of variable spacingof identical subunits. Piping and control are not shown.

[0016]FIG. 6 is a graphical depiction of an ideal transverse profile ofheat flux of the reformer of FIG. 1.

[0017]FIG. 7 is a graphical depiction of an ideal longitudinal profileof heat flux in the reformer of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention is directed to a novel burner design for afurnace whereby specified heat flux profiles in two dimensions (e.g.,along a burner longitudinal axis and along a burner transverse axis) areachieved simultaneously. A furnace to which the present invention isapplied has one or more burner assemblies. Each burner assembly consistsof a number of burner subunits that share the same air supply, fuelsupply and control system. The number and size of air and fuel orificesin each burner subunit control the transverse profile of the flamewithin the burner, the spacing among the burner units controls thelongitudinal profile of the flame within the burner, and a specialair-fuel mixing approach ensures that the heat flux profiles maintainthe same shape at different fuel and air input rates.

[0019] For purposes of the present invention, the term “longitudinal”refers to the longitudinal axis of the burner and the term “transverse”refers to axes perpendicular to the longitudinal axis of the burner.

[0020] To achieve the objectives of this invention, three principles areused together to create a novel design apparatus.

[0021] First, the heat flux profile requirement for the particularfurnace is reduced into solvable sub-problems by physical subdivision.The required heat release is provided in the form of fuel to meet thetargeted heat transfer requirement in each subdivision. This principleis applied to the longitudinal heat flux profile (i.e., a heat fluxprofile with respect to the longitudinal axis of the burner), which isachieved through the use of a plurality of subunits within the burnerassembly. These subunits may be: (1) subunits having the same heatrelease rate and placed at a variable spacing, (2) subunits havingdifferent heat release rates and placed at a fixed spacing, or (3) acombination of (1) and (2) above. This principle can also be applied atthe level of each subunit so that a plurality of flamelets, eachresponsible for a prescribed target area of heat transfer, collectivelyachieves a desired transverse heat flux profile at each elevation.

[0022] Second, it is known that the length of a turbulent flame isdirectly proportional to its nozzle (orifice) diameter. See, forexample, J. M. Beer and N. A. Chigier, Combustion Aerodynamics, JohnWiley and Sons, New York, 1972 at page 40. See also H. Tennekes and J.L. Lumley, A First Course in Turbulence, The MIT Press, 1990 at page 22.This principle is used to control the length of the flamelets withineach subunit of the burner assembly so that the desired amount of energyis delivered to the target location at a given distance away from thesubunit. Accordingly, more orifices of smaller diameters will produce ashorter flamelet. Conversely, fewer orifices of larger diameters willproduce a longer flamelet. This principle is directed to the transverseheat flux profile of the furnace (i.e., the heat flux profile of thefurnace of a plane that is perpendicular to the longitudinal heat fluxprofile of the furnace).

[0023] Third, proper air-fuel ratios are maintained and air staging isused to control flame temperature. Although a premixed design may offercertain performance benefits, safety requirements may favor anon-premixed approach. Whether premixed or non-premixed, proper air-fuelmixing is critical to achieving flame shape and heat flux profiles.Furthermore, in a non-premixed design, not only must the overallfuel-ratio be correct, ratios within each subdivision must also becarefully controlled so that the primary stage, secondary stage, etc.,all have proper stoichiometries. In addition, the intersection points offuel and air jets must be properly controlled.

[0024] The objective of low capital cost is achieved by consolidatingthe flow manifolds and burner controls. Regardless of the number ofsubunits in the assembly of the present invention, there is only one aircontrol valve and one fuel control valve. The proper distribution of airand fuel is achieved by appropriately sizing air ducts and fuel pipes.

[0025] Cylindrical Steam Hydrocarbon Reformer

[0026] Referring now to the drawings, wherein like part numbers refer tolike elements throughout the several views, there is shown in FIG. 1 acylindrical steam reformer 10 designed in accordance with the presentinvention. This reformer 10 may be, for example, a reformer as describedin a U.S. application Ser. No. 09/741,284, filed Dec. 20, 2000, andentitled Reformer Process with Variable Heat Flux Side-Fired BurnerSystem, the complete specification of which is hereby fully incorporatedby reference.

[0027] The steam reforming process is a well known chemical process forhydrocarbon reforming. Typically, a hydrocarbon and steam mixture (amixed feed) reacts in the presence of a catalyst to form hydrogen,carbon monoxide, and carbon dioxide. Since the reforming reaction isstrongly endothermic, heat must be supplied to the reactant mixture,such as by heating the tubes in a furnace or reformer. The amount ofreforming achieved depends on the temperature of the gas leaving thecatalyst. Exit temperatures of 700 to 900 degrees Celsius are typicalfor hydrocarbon reforming.

[0028] As can be seen in FIGS. 1 and 2, the reformer 10 of this exampleof the present invention includes a cylindrically shaped, refractorylined shell 12. Multiple burner subunits 14 are located along the innerwall 16 of the shell 12. At the upper end 18 of the shell 12, there areone or more openings 20 that allow the flue gas (containing combustionproducts) to flow from the shell 12. Conventional reformer tubes 22containing catalyst are positioned within the interior of the shell 12to utilize high intensive radiant heat directly from the flames of theburner subunits 14. Fuel supply 17, air supply 19, and control system 21are also shown in schematic form.

[0029] The cylindrical reformer 10 requires burner subunits 14 thatproduce a specified heat flux along each reformer tube 22 (i.e., alongitudinal heat flux profile), and, at any given elevation of thereformer tube 22, the heat flux profile must be uniform among a numberof tubes 22 (i.e., the transverse profile).

[0030] As can be seen in FIG. 2, The cylindrical reformer 10 of thisexample is divided into a plurality of pie-shaped sectors 24, here, sixsectors. Each sector 24 requires a burner assembly (that includes burnersubunits 14) that is mounted on the inner wall 16 of the shell 12 alongthe length of shell 12. The burner subunits 14 are fired horizontallyand radially in an inward direction. This arrangement requires theburner subunits 14 to produce a uniform heat flux at a given elevationon the sides of the sector where reformer tubes 22 are installed inradial rows 30. The flame must be compact to avoid local hot spots.Furthermore, the process requires an optimum heat flux profile alongeach reformer tube 22, generally known in the catalytic steam methanereforming art.

[0031] These two heat flux profile requirements limit the flame of eachsubunit 14 to a fan shape 38 (see FIG. 3). The burner subunits 14 mustoperate for a range of fuels and air preheat temperatures.

[0032] As seen in FIG. 3, to achieve a uniform heat flux radially at agiven elevation, the total heat release from a burner subunit 14 can bedivided into arrays of flamelets 26 that create the fan shape 38, eachof which aims at a given cluster of reformer tubes 22, which are thetargets of heat transfer 32. If, for example, each flamelet 26 is tocover the same heat transfer surface area, the heat release for eacharray must be uniform. That is, the fuel supply used to create the fanshape is identical. In addition, the distance from the burner subunit 14(which, in this example, is mounted at the center of the sector on thesidewall) to each of the cluster of reformer tubes 22 (i.e., the targetof heat transfer 32) is not uniform because of the pie-shaped geometry.For the heat transfer to each cluster of tubes to be uniform, thesubunit 14 must produce different flame lengths for different flamelets26. This requirement is achieved through the use of variable orificesizes.

[0033] In this example reformer 10, there are six pie-shaped sectors 24and seven reformer tubes 22 along each radial row 30 of reformer tubes22 that divide the sectors 24. FIG. 3 depicts one of the six pie-shapedsectors 24. As indicated above, the reformer tubes 22 are preferablyarranged uniformly along the radial rows 30. Here, it is desired thatthe burner subunit 14 be constructed to provide seven flamelets 26, sothat each flamelet 26 covers a pair of reformer tubes 22. As shown inFIG. 3, the flamelet angles are approximately 30, 50, 70, 90, 110, 130,and 150 degrees, and the heat release is preferably approximately equalfor each of the seven flamelets 26. Due to symmetry, flamelets 26 ateach of 30 & 150 degrees, 50 &130 degrees, and 70 & 110 degrees musthave substantially the same profile. Also as shown, the distance to eachof the desired heat transfer target 32 varies due to the cylindricalgeometry. If it is assumed that the distance for the 30-degree flameletis 1 unit based on this geometry, the distance for the 50 degreeflamelet is 1.08 units, the distance for the 70 degree flamelet is 1.32units, and the distance for the 90 degree flamelet is 1.89. Thisarrangement is shown in FIG. 3.

[0034] Based on the geometry indicated in the preceding paragraph, andthe fact that flow rate is proportional to orifice cross-sectional area,the following relationships are derived from the design principlesdisclosed here:

n₁d₁ ²=n₂d₂ ²=n₃d₃ ²=n₄d₄ ²= . . . =n_(i)d_(i) ²$\frac{L_{1}}{d_{1}} = {\frac{L_{2}}{d_{2}} = {\frac{L_{3}}{d_{3}} = {\frac{L_{4}}{d_{4}} = {\ldots = \frac{L_{i}}{d_{i}}}}}}$

[0035] where n is number of orifices in each angle (e.g., the 30 degreeangle, the 50 degree angle, the 70 degree angle, etc.), d is orificediameter (see FIG. 4), and L is length from the burner subunit to thetube row 30 in each angle (See FIG. 3). In this example, the first angleis at 30 degrees (subscript 1), the second angle is at 50 degrees(subscript 2), and so forth. Description of only four angles is neededfor a complete description of the system because of symmetry. To controlthe lengths of the flamelets 26 for associated heat transfer targetareas 32, the air orifice arrangement 34 (see FIG. 4) can be calculatedusing these formulas. FIG. 4 depicts a quarter of the burner subunit 14face and shows the air orifice arrangement 34 and the fuel tiparrangement 36.

[0036] Ignition air orifices 35 are also shown. The remaining threequarters have the same configuration due to symmetry. Here, it isrecognized that the air jet momentum overwhelms fuel jet momentum inair-fuel combustion, therefore variable orifice sizes for fuel aregenerally unnecessary. It is also clear to those skilled in the art thatthese orifices can be for a premixed oxidant-fuel mixture rather thanoxidant alone. Furthermore, if the fuel momentum is significant, such asin cases of low-heating-value fuels, a similar arrangement can bedevised for the fuel orifices as well. It is noted that, to this point,the first and second principles, as described above, have been applied.

[0037] To ensure flame stability and to achieve a desired flame shape,the third principle above, i.e., proper air-fuel ratios, must be appliedin arranging the air orifices. Industry guidelines on the ratio ofprimary air to total air is usually between 40 to 60%, but the ratiocould be as low as about 25%, or as high as about 75%. As FIG. 4suggests, which depicts the air orifice arrangement 34 and fuel orificearrangement 36 of one quarter of a burner subunit 14, the amount ofprimary air stays within that guideline. Note that the burner subunit 14is symmetric about both the X and Y axes shown. The orifice arrangementhere achieves variable lengths of flamelets 26. FIG. 4 also showsorifices for ignition air flow 35 as a further measure to ensure flamestability.

[0038] The desired longitudinal heat flux profile can be achieved byarranging the burner subunits 14 in a manner similar to that of FIG. 5,which illustrates variable spacing with identical subunits. It isrecognized that it is possible to use variable spacing or variable heatrelease capacity, or a combination thereof, to achieve the same result.

[0039] Prototype Test Data

[0040] One burner assembly consisting of six subunits was constructedand tested in a vertical cylindrical furnace. At each subunit elevation,five heat flux samples were taken. FIG. 4 shows the ideal transverseheat flux profile, which was substantially confirmed by prototype testdata. FIG. 5 shows the ideal longitudinal heat flux profile that wasalso substantially confirmed by the prototype test data.

[0041] It is clear to those skilled in the art that if the number ofheat transfer targets and/or the furnace geometry is different, the samedesign approach can be used to come up with a design that will achievethe same objective.

[0042] As long as the heat flux profiles required by the process areknown, this design approach can be used to design a burner assembly tomeet those requirements. As a result, this invention can haveapplications far beyond the embodiment described herein.

[0043] Separately, the three principles of burner design discussedherein are known. It is the application of the combination of theseprinciples that is novel. The net outcome is a low-cost burner assemblythat satisfies heat flux profile requirements in two orthogonaldimensions simultaneously. Such a burner has wide applicability indifferent industries, such as hydrogen reformers, ethylene crackers,process heaters, utility boilers, and the like. The key to low cost isthe consolidation of flow distribution and burner control.

[0044] Although illustrated and described herein with reference tospecific embodiments, the present invention nevertheless is not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims without departing from the spirit of the invention.

1. A burner comprising a plurality of burner subunits, said burnersubunits sharing a single air supply, a single fuel supply and a singlecontrol system, each burner subunit having a plurality of air orificesand a plurality of fuel orifices, said plurality of air orifices andsaid plurality of fuel orifices being of sufficient quantity and eachsaid air orifice and each said fuel orifice being of a cross-sectionalarea to control a transverse heat flux profile of said burner, saidburner subunits being spaced with respect to one another to control alongitudinal heat flux profile of said burner, said single air supplyand said single fuel supply adapted to provide an air-fuel mix thatensures the transverse heat flux profile and the longitudinal heat fluxprofile are maintained at different fuel and air input rates.
 2. Theburner of claim 1, wherein each of said plurality of burner subunits isspaced at variable spacing with respect to one another to control saidlongitudinal heat flux profile.
 3. The burner of claim 1, wherein eachof said plurality of burner subunits is spaced at a constant distancewith respect to one another to control said longitudinal heat fluxprofile.
 4. The burner of claim 1, wherein each of said plurality ofburner subunits is spaced at either variable spacing or constant spacingwith respect to one another to control said longitudinal heat fluxprofile.
 5. The burner of claim 1, wherein each of said plurality ofburner subunits has a plurality of air orifices of a desiredcross-sectional area, each air orifice adapted to create a flamelet tocontrol said transverse heat flux profile of said burner.
 6. A burnercomprising a plurality of burner subunits, said burner subunits sharinga single air/fuel supply and a single control system, each burnersubunit having a plurality of air/fuel orifices, said plurality ofair/fuel orifices being of sufficient quantity and each air/fuel orificebeing of a cross-sectional area to control a transverse heat fluxprofile of said burner, said burner subunits being spaced with respectto one another to control a longitudinal heat flux profile of saidburner, said air/fuel supply adapted to provide an air-fuel mix thatensures the transverse heat flux profile and the longitudinal heat fluxprofile are maintained at different fuel and air input rates.
 7. Theburner of claim 6, wherein each of said plurality of burner subunits isspaced at variable spacing with respect to one another to control saidlongitudinal heat flux profile.
 8. The burner of claim 6, wherein eachof said plurality of burner subunits is spaced at a constant distancewith respect to one another to control said longitudinal heat fluxprofile.
 9. The burner of claim 6, wherein each of said plurality ofburner subunits is spaced at either variable spacing or constant spacingwith respect to one another to control said longitudinal heat fluxprofile.
 10. The burner of claim 6, wherein each of said plurality ofburner subunits has a plurality of air/fuel orifices of a desiredcross-sectional area, each air/fuel orifice adapted to create a flameletto control said transverse heat flux profile of said burner.